1. Field of the Invention
The present invention relates to a nonaqueous electrolyte secondary battery. More specifically, the invention relates to a nonaqueous electrolyte secondary battery in which a void volume of an active material layer per battery capacity is adjusted.
2. Description of Related Art
A nonaqueous electrolyte secondary battery such as a lithium ion secondary battery has a lighter weight and higher energy density than those of existing batteries. Therefore, the nonaqueous electrolyte secondary battery is suitably used as a driving power supply used for a vehicle such as an electric vehicle or a hybrid vehicle or as a so-called portable power supply used for a personal computer, a portable device, or the like.
In such a typical configuration of the nonaqueous electrolyte secondary battery, a positive electrode and a negative electrode include active material layers containing, as a major component, materials (active materials) that can reversibly store and release charge carriers. Voids are formed in the active material layers. The battery is charged and discharged by the charge carriers moving between the active materials of the positive and negative electrodes through a nonaqueous electrolytic solution impregnated into the voids. Accordingly, the amount and properties (for example, a pore size) of the voids in the active material layers have significant effects on battery characteristics. For example, Japanese Patent Application Publication No. 2007-207535 (JP 2007-207535 A) describes that, by increasing a porosity (VC) of a positive electrode active material layer to be higher than a porosity (VA) of a negative electrode active material layer, the movement of charge carriers is well-balanced between positive and negative electrodes, and superior output performance can be obtained.
In some cases, a nonaqueous electrolyte secondary battery is used while repeating rapid charging and discharging in which a high current instantaneously flows at a high rate of, for example, 2 C (particularly, 5 C) or higher. In this case, the battery is used as a vehicle-mounted power supply with high output. In the battery, a load which is applied to an active material layer along with the movement of charge carriers is higher than that of, for example, the battery described in JP 2007-207535 A. Specifically, when the battery is continuously charged and discharged at a high rate, an excess amount of nonaqueous electrolytic solution may be extruded from an active material layer due to, for example, an effect of a pumping action working along with the expansion and contraction of an active material. Accordingly, the density of charge carriers in the nonaqueous electrolytic solution may be non-uniform, or charge carriers may be concentrated on one electrode side. As a result, a load applied along with the movement of the charge carriers increases, and thus an internal resistance may increase.